java - Regex to match a C-style multiline comment

Question

I have a string for e.g.

String src = "How are things today /* this is comment **/ and is your code  /** this is another comment */ working?"

I want to remove /* this is comment **/ and /** this is another comment */ substrings from the src string.

I tried to use regex but failed due to less experience.

Answer 1

The best multiline comment regex is an unrolled version of (?s)/*.*?*/ that looks like

String pat = "/\*[^*]*\*+(?:[^/*][^*]*\*+)*/";

See the regex demo and explanation at regex101.com.

In short,

• /* - match the comment start /*
• [^*]**+ - match 0+ characters other than * followed with 1+ literal *
• (?:[^/*][^*]**+)* - 0+ sequences of:
  • [^/*][^*]**+ - not a / or * (matched with [^/*]) followed with 0+ non-asterisk characters ([^*]*) followed with 1+ asterisks (*+)
• / - closing /

David's regex needs 26 steps to find the match in my example string, and my regex needs just 12 steps. With huge inputs, David's regex is likely to fail with a stack overflow issue or something similar because the .*? lazy dot matching is inefficient due to lazy pattern expansion at each location the regex engine performs, while my pattern matches linear chunks of text in one go.